The present invention relates to improvements in the electrodeposition of zinc from aqueous acid plating baths, not taught or suggested by the prior art, yielding the elimination of ammonia as a by-product discharged to waste treatment facilities, and yielding a synergistic result by producing bright deposits at high current densities. Non-ammoniated acidic zinc plating baths have been known, and these baths can be made to produce a bright electrodeposit of zinc. These baths, however, inherently produce bright electrodeposits over rather narrow current density ranges. The most predominant problem with these baths has been their inability to work well at the high current densities encountered at the edges of parts in situations where an excessive amount of current is required to obtain deposits in hard to plate recessed areas. Instead of being bright, the edges of the parts are plated with a dull, coarse deposit of zinc.
The use of ammonium chloride by those skilled in the art tends to alleviate these high current density problems, but new problems are created. The presence of ammonia is undesirable because of the biochemical oxygen demand caused by ammonia and the chlorine demand of the ammonia in waste treatment facilities. Additionally, certain complexing agents such as hydroxy carboxylic acids and ethylenediamine tetraacetic acid have been used to improve the quality of the high current density deposits. However, some of these complexing agents along with ammonia can make the precipitation of zinc during waste treatment very difficult, if not impossible.
The acid zinc bath formulations including ammonium ions as a bath constituent fail to address the ammonia effluent problem. U.S. Pat. No. 3,723,263, and a division thereof, U.S. Pat. No. 3,767,540, both relate to aqueous acid zinc electroplating baths containing naphthol polyoxyalkylate in combination with polyethyleneimine. These patents also state that aromatic aldehydes and ketones may be present. U.S. Pat. No. 3,729,394 relates to an acid zinc electroplating bath having dissolved therein a block copolymer of propylene oxide and ethylene oxide. The block copolymer may be utilized alone or in instances with a compound of N-(alkyl sulfonyl) glycine. Additionally, another U.S. patent, namely U.S. Pat. No. 2,674,619, relates to polyoxyalkylene compounds which have an outstanding detergent and surface agent properties and in which the hydrophobic element is a polyoxypropylene polymer having a molecular weight of at least 900.
Therefore, the need exists for a brightening agent which is capable of providing brightening effects over a broad range of electrodeposition current densities, while avoiding ammonia as a byproduct in such concentrations to excessively complicate waste treatment of the effluent from the plating bath.